This invention relates to a process for underground retorting of oil shale.
Researchers have now renewed their efforts to find alternate sources of energy and hydrocarbons in view of past rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts or in underground in situ retorts. In situ retorts require less mining and handling than surface retorts.
In vertical in situ retorts, a flame front moves downward through a rubblized bed containing rich and lean oil shale to liberate shale oil, off gases and condensed water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, none of the shale is mined, holes are drilled into the formation and the oil shale is explosively rubblized, if necessary, and then retorted. In modified in situ retorts, some of the oil shale is removed by mining to create a cavity which provides extra space for explosively rubblized oil shale. The oil shale which has been removed is conveyed to the surface and retorted above ground.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Colorado Mahogany zone oil shale contains several carbonate minerals which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23% dolomite (a calcium/magnesium carbonate) and about 16% calcite (calcium carbonate), or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8% of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zones. The choice of a particular retorting method must therefore take into consideration carbonate decomposition as well as raw and spent materials handling expense, product yield and process requirements.
While efforts are made to explosively rubblize the oil shale into uniform pieces, in reality the rubblized mass of oil shale contains numerous different sized fragments of oil shale which create vertical, horizontal and irregular channels extending sporadically throughout the bed and along the wall of the retort. As a result, during retorting, hot gases often flow down these channels and bypass large portions of the bed, leaving significant portions of the rubblized shale unretorted.
Different sized oil shale fragments, channeling and irregular packing, and imperfect distribution of oil shale fragments cause other deleterious effects including tilted (nonhorizontal) and irregular flame fronts in close proximity to the retorting zone and fingering, that is, flame front projections which extend downward into the raw oil shale and advance far ahead of other portions of the flame front. Irregular flame fronts and fingering can cause coking, burning, and thermal cracking of the liberated shale oil. Irregular, tilted flame fronts can lead to flame front breakthrough and incomplete retorting. In the case of severe channeling, horizontal pathways may permit oxygen to flow underneath the raw unretorted shale. If this happens, shale oil flowing downward in that zone may burn. It has been estimated that losses from burning in in situ retorting can be as high as 40% of the product shale oil.
Furthermore, during retorting, significant quantities of oil shale retort water are also produced. Oil shale retort water is laden with suspended and dissolved impurities, such as shale oil and oil shale particulates ranging in size from less than 1 micron to 1,000 microns and contain a variety of other contaminants not normally found in natural petroleum (crude oil) refinery waste water, chemical plant waste water or sewage. Oil shale retort water usually contains a much higher concentration of organic matter and other pollutants than other waste waters or sewage causing difficult disposal and purification problems.
The quantity of pollutants in water is often determined by measuring the amount of dissolved oxygen required to biologically decompose the waste organic matter in the polluted water. This measurement, called biochemical oxygen demand (BOD), provides an index of the organic pollution in the water. Many organic contaminants in oil shale retort water are not amenable to conventional biological decomposition. Therefore, tests such as chemical oxygen demand (COD) and total organic carbon (TOC) are employed to more accurately measure the quantity of pollutants in retort water. Chemical oxygen demand measures the amount of chemical oxygen needed to oxidize or burn the organic matter in waste water. Total organic carbon measures the amount of organic carbon in waste water.
Over the years, a variety of methods have been suggested for purifying or otherwise processing oil shale retort water. Such methods have included shale adsorption, in situ recycling, electrolysis, flocculation, bacteria treatment and mineral recovery. Typifying such methods and methods for treating waste water from refineries and chemical and sewage plants are those described in U.S. Pat. Nos. 2,948,677; 3,589,997; 3,663,435; 3,904,518; 4,043,881; 4,066,538; 4,069,148; 4,073,722; 4,124,501; 4,178,039; 4,121,662; 4,207,179; and 4,289,578. Typifying the many methods of in situ retorting are those found in U.S. Pat. Nos. 1,913,395; 1,191,636; 2,418,051; 3,001,776; 3,586,377; 3,434,757; 3,661,423; 3,951,456; 3,980,339; 3,994,343; 4,007,963; 4,017,119; 4,105,251; 4,120,355; 4,126,180; 4,133,380; 4,149,752; 4,153,300; 4,158,467; 4,117,886; 4,185,871; 4,194,788; 4,199,026; 4,210,867; 4,210,868; 4,231,617; 4,243,100; 4,263,969; 4,263,970; 4,265,486; 4,266,608; 4,271,904; 4,315,656; 4,323,120; 4,323,121; 4,328,863; 4,343,360; 4,343,361; 4,353,418; 4,378,949; 4,425,967; and 4,436,344. These prior art processes have met with varying degrees of success.
It is, therefore, desirable to provide an improved in situ oil shale retort and process which overcome most, if not all, of the above problems.